Like all blood cells, T lymphocytes are constantly lost during life, and must be continuously replaced. The thymus is the primary site of T lymphopoiesis. However, the thymus is characterized by an accelerated, age-related atrophy, leading to a progressive decline in new (na?ve) T lymphocyte production with age. Although replication of pre-existing T lymphocytes prevents the appearance of lymphopenia, these are mostly memory T cells. Consequently, the ability to respond to new immunological challenges, such as emerging viral strains, decreases with age, as does response to vaccination. The mechanisms for accelerated thymic aging are not well resolved, but it is now clear that it is primarily a stromal phenotype. We have recently published a novel approach for the characterization of thymic stromal gene expression in situ, providing an unprecedented view of stromal biology, and revealing many insights into their nature. One unexpected finding was that thymic stromal cells are deficient in key enzymes involved in protection from damage caused by reactive oxygen species (ROS), as well as several enzymes that repair that damage. Most notably, thymic stromal cells appear to be profoundly deficient in catalase, a major scavenger of H2O2. Since thymic stromal cells are relatively long-lived, and exist in a microenvironment characterized by intense metabolic activity (associated with lymphoblastic progenitor proliferation), defects in protection from / repair of ROS damage would be expected to result in an accelerated decline in function, viability, and replicative ability, and the appearance of atrophy. Our central hypothesis is that epithelial stromal cells of the thymus are exquisitely sensitive to ROS by virtue of these enzyme deficiencies, and that thymic atrophy is a consequence of accelerated accumulation of metabolic damage. In this application, we present powerful biochemical and genetic data to support this hypothesis, and propose experiments to further prove it, including showing that genetic replacement of catalase specifically in epithelial stroma of the thymus is sufficient to prevent atrophy, that epithelial stromal cells of the thymus accumulate more damage in vivo than other age- and/or lineage-matched cells, that isolated thymic epithelial cells are more sensitive to ROS than other age- and/or lineage-matched cells, and that deficiencies in repair mechanisms exacerbate the consequences of catalase deficiency in thymic epithelial cells. These studies are expected to reveal the mechanism of accelerated thymic atrophy, and may suggest approaches to reduce or prevent this pathological state.